The bone loss of old age is independent of the loss of sex steroids, and is due to a decrease in the number of osteoblasts. This decline must be due to derangements in the mechanisms controlling osteoblast number including the magnitude of progenitor proliferation, the prevalence of apoptosis, and the fraction of multi-potential progenitors committed to the osteoblast lineage. During the previous funding period, it was found that the self-renewal of early osteoblast progenitors (CFU-OB) is reduced in the SAMP6 murine model of age-related osteopenia, and in old C57/BL/6 mice. Because of its logarithmic nature, small changes CFU-OB self- renewal may profoundly affect osteoblast production. It was also demonstrated that apoptosis is the fate of most osteoblasts. By analogy with other regenerating tissues, osteoblast progenitors may also die by apoptosis. Further, the increased adipocyte differentiation seen in SAMP6 mice might occur at the expense of osteoblasts because of the common lineage of these cell types. This may be explained by increased synthesis and/or activation of the adipocyte transcription factor PPARgamma2, which was found to suppress osteoblast differentiation. The finding of two mechanisms leading to bone loss in SAMP6 mice is consistent with the polygenic basis of osteopenia in this model, and the multi-factorial basis of age-related diseases. These observations form the basis of the hypothesis that the age-related reduction in osteoblast number is due to derangement of at least 3 distinct mechanisms that govern osteoblast and osteoblast progenitor fate: a) reduced self-renewal of early mesenchymal progenitors, b) increased apoptosis, and c) increased PPARgamma2- induced differentiation of mesenchymal progenitors, b) increased apoptosis, and c) increased PPARgamma2-induced differentiation of mesenchymal progenitors into adipocytes at the expense of osteoblasts. To test this hypothesis, the relationship between the decline in CFU-OB self-renewal and the reduction of osteoblast number in SAMP6 mice and aged C57/BL/6 mice will be established; and the biochemical basis of the decreased self-renewal will be studied. In addition, the role of osteoblast progenitor and/or osteoblast apoptosis in determining osteoblast number will be studied by over-expressing the anti-apoptotic protein bcl-2 in these cells in transgenic mice. Finally, the role of PPARgamma2 in the regulation of osteoblast differentiation will be elucidated in vitro and in vivo using specific activators and antagonists of this transcription factor.